Power is the rate at which work is done or energy is transferred. In simple terms, it tells us how fast energy is being used or produced.
Mathematically, power is expressed as:
\$P = \frac{W}{t}\$
Where \$W\$ is the work done (in joules) and \$t\$ is the time taken (in seconds). The SI unit of power is the watt (W), where 1 W = 1 J s⁻¹.
Think of power like the flow of water through a pipe. The amount of water (energy) that passes a point per second (time) is the power. A larger pipe or higher pressure (force) means more power.
A 60 W bulb uses 60 joules of energy every second. If you run it for 2 hours (7200 s), the total energy used is:
\$E = P \times t = 60\,\text{W} \times 7200\,\text{s} = 432{,}000\,\text{J}\$
| Device | Power (W) | Time (s) | Work Done (J) |
|---|---|---|---|
| Flashlight | 5 | 120 | \$5 \times 120 = 600\$ |
| Laptop | 45 | 3600 | \$45 \times 3600 = 162{,}000\$ |
Tip: When you see a question asking for power, remember the formula \$P = \frac{W}{t}\$. If the problem gives force and distance, first calculate work \$W = F \cdot d\$, then divide by time.
Also, check units carefully: power should be in watts (J s⁻¹). If you get joules or seconds alone, you need to divide or multiply accordingly.
A 100 W light bulb is switched on for 3 hours. How much energy does it consume?
Solution: \$E = P \times t = 100\,\text{W} \times (3 \times 3600\,\text{s}) = 1{,}080{,}000\,\text{J}\$.